An atmospheric pressure chemical ionization triple quadrupole mass spectrometer has been coupled to a smog chamber to study the products of atmospherically important hydrocarbon oxidation reactions. Traditional MS and MS/MS scan modes were used to identify ion signals arising from possible reaction products and Multiple Reaction Monitoring (MRM) was used to follow a number of these as target→fragment ion pairs over the course of the reaction. Mechanistic information has been inferred from the time dependence of product signals. MRM profiling has allowed identification of interferences that occur due to isobaric ions resulting from the formation of isobaric products and/or clustering, which can be undetectable using the MS and MS/MS modes. Differences in product formation rate results in variations of their MRM ion pair onset and time profile therefore allowing separation to be observed. This method was tested during a study of the products of the HO radical oxidation of β-pinene. The oxidation product pinaketone (MW 138) has been shown to have interferences at its (M + H)+, m/z 139, being more accurately monitored using its (M + H + H2O)+ cluster ion pairs, 157→139 and 157→121. Furthermore, the time dependence of the ion pair 157→111 has lead to identification of a more highly oxidized acid-aldehyde product. It has been determined that an organic nitrate contributing to m/z 216 based on its time dependence relative to pinaketone cannot be the expected simple C10 hydroxynitrate product but rather a more highly oxidized C9 nitrate. Using MRM to follow ion signals as a function of reaction time has proven to be a valuable addition to existing mass spectrometric acquisition modes for reaction product determination.